An "Autobuild" version is a version of CHDK that is still under development. As the developers compile new versions to test, their efforts are "automatically built" and made available on the web for beta testing. As with most beta software, not all new features have been fully tested. Some features may not work with all camera models. Your feedback and bug-reports concerning these new builds are welcome on the CHDK forum or Bugtracker.
Newcomers may find it more comfortable to not use an Autobuild, unless it is necessary to run CHDK on your camera. Although CHDK does not make any changes to the actual firmware on your camera, it may be possible to cause mechanical damage to your camera, but this is extremely unlikely due to the safeguards put in place by Canon - most people prefer not to work with software that hasn't been thoroughly debugged.
For further information on CHDK, see here